One knows that the hydraulic jacks consist in general of a rod which pushes a piston in a cylinder. But the bulkiness exhibited by such jacks is about twice the length of the rod thereby constituting an inconvenience when one must use them in an apparatus having necessarily to exhibit a very small bulkiness.
On the other hand one has already proposed systems in which an externally threaded rod extends through a piston held immovable against rotation so that the rotation of the rod permits the translatory motion of the piston along the said rod.
But such a system is not free of drawbacks, in particular in connection with the fluid-tightness between rod and piston, which fluid-tightness often is imperfect let apart the possible fouling of the system.